Riddles of the moon sides deciphered?

The lunar hemispheres are very different from each other. A new simulation now provides an explanation for this mystery of lunar geology.

Providence (USA). The stark differences in the lunar hemispheres are among the greatest mysteries of the astronomy. The side facing away from the earth moon has a thick, very old crust, the terrestrial side is thinner and covered with dark, solidified lava. In addition, the crust of the front has a high content of rare earth elements, potassium, phosphorus and radioactive thorium.

How the volcanism on the front side of the moon and the KREEP terrain there developed is shown in the research controversial. Some planetary scientists have theorized that Earth’s gravity caused the crustal differences. However, other scientists are of the opinion that an impact on the side of the moon facing us caused the differences.

Explanation for differences in the lunar hemispheres

A team of BrownUniversity has now published an alternative explanation for the differences in the lunar hemispheres. The scientists led by Matt Jones analyzed the more than 2,000-kilometer-wide South Pole-Aitken Basin, the largest impact crater of the moon. This was formed about 4.3 billion years ago when the moon fell from a planet about 100 kilometers in size asteroids was hit. The South Pole-Aitken Basin lies opposite the KREEP terrain.

“We know that large impacts like the South Pole-Aitken Basin generate a lot of heat. The question is how this heat has affected the dynamics of the lunar interior,” explains Jones. Like the scientists in the specialist magazine Science Advances published, they therefore reconstructed the effects of the collision using geophysical models.

Thermal anomaly in the lunar mantle

They were able to determine that “the South Pole impact caused a thermal anomaly in the lunar mantle that shaped the development of the moon’s interior for hundreds of millions of years.” region of KREEP terrain, reached.

“This heat swell triggered currents in the lunar mantle that swept away KREEP and titanium-rich magma cumulates and washed them toward the moon’s side facing us,” the authors said. Elements and minerals followed the flow of heat. According to the simulations, the heat of the impact and the subsequent mantle convection were sufficient for the element enrichments in the KREEP terrain.

Volcanism as a late consequence of the asteroid

In addition, the asteroid impact could also the lunar volcanism triggered. “Under all plausible conditions of the time, this event also concentrated heat-generating elements on the face of the moon,” explains Jones. This, combined with mantle convection and lunar radioactive heating, could have triggered intense heat in the lunar upper mantle for hundreds of millions of years.

Thus, hot magma could rise and trigger large eruptions. “The first marebasalts on the side of the moon facing us erupted around 200 million years after the impact, and the most intense phase of mare volcanism followed around 500 to 700 million years later,” the authors report.

Chain reaction from impact on the moon

According to the researchers, the reconstruction thus proves that the geochemical anomalies of the KREEP terrain and the South Pole-Aitken Basin are linked. Both could have been caused by a chain reaction triggered by the asteroid’s impact.

“How the KREEP terrain came about is certainly one of the most important open questions in lunar research. And the impact that created the South Pole-Aitken Basin is one of the most significant events in lunar history. Our work now brings the two together and is delivering some pretty exciting results,” Jones states.

Science Advances, doi: 10.1126/sciadv.abm8475